Method for forming MTJS with lithography-variation independent critical dimension

ABSTRACT

Some examples relate to a method for forming a semiconductor device. The method comprises forming a pattern definition stack over a substrate, the pattern definition stack comprising a transfer layer, an interlayer arranged over the transfer layer, and a patterning layer arranged over the interlayer. The method further comprises forming a first opening in the patterning layer to expose an upper surface of the interlayer and etching the interlayer with an at least partially isotropic etchant through the first opening to form a recessed cavity. The method further comprises forming a conformal layer over the interlayer and the patterning layer to fill the first opening, and etching the conformal layer and the transfer layer with an anisotropic etch to form a second opening in the transfer layer. The method also comprises depositing a hard mask material in the second opening.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/927,890, filed on Oct. 30, 2019, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Electronic memory is ubiquitous in modern electronic devices. Ingeneral, electronic memory allows the storage and read-out ofinformation with electronic control and can be divided into volatilememory and non-volatile memory. Non-volatile memory is able to retainits stored data in the absence of power, whereas volatile memory losesits stored data when power is lost. Magnetoresistive random-accessmemory (MRAM) is one promising candidate for next generationnon-volatile electronic memory due to advantages over current electronicmemory regarding power consumption, durability or scalability.

An MRAM cell for storing information includes a magnetic tunnel junction(MTJ) structure, and a resistance of the MTJ structure is adjustable torepresent logic “0” or logic “1”. The MTJ structure includes onemagnetic reference layer and one ferromagnetic free layer separated by atunneling insulating layer, typically termed a “tunnel junction”. Theresistance of the MTJ element is adjusted by changing a direction of themagnetization of the ferromagnetic free layer with respect to that ofthe reference layer. Depending on the relative alignment of themagnetization in the free layer and the reference layer, transmission ofelectrons through the tunnel junction is increased or decreased. Theresulting low and high resistances are utilized to indicate a digitalsignal “0” or “1”, thereby allowing for data storage and read out in anMRAM cell. As the information is encoded in the magnetization, it can bestored over long periods of time without expending electrical energy,allowing for devices having lower power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some examples of themagnetic cell part of an MRAM cell, including a magnetic tunnel junction(MTJ), according to the present disclosure.

FIG. 2A, 2B illustrates schematic views of some examples of anintegrated circuit including the magnetic cell part of an MRAM cellcombined with one or more transistors, which function as a selector usedfor addressing the cell.

FIGS. 3 through 14 illustrate a series of incremental manufacturingsteps as a series of cross-sectional views.

FIGS. 15 through 17 illustrate a series of incremental manufacturingsteps as a series of cross-sectional views, complemented by a schematicperspective view in FIG. 16B.

FIGS. 18 through 21 illustrate a series of incremental manufacturingsteps as a series of cross-sectional views.

FIG. 22 illustrates a methodology in flowchart format that illustratessome examples of the present concept.

DETAILED DESCRIPTION

The present disclosure provides many different examples for implementingdifferent features of this disclosure. Specific examples of componentsand arrangements are described below to simplify the present disclosure.These are, of course, merely examples and are not intended to belimiting. For example, the formation of a first feature over or on asecond feature in the description that follows may include examples inwhich the first and second features are formed in direct contact, andmay also include examples in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact. In addition, the presentdisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition is for the purpose of simplicity and clarityand does not in itself dictate a relationship between the variousexamples and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

An MRAM magnetic cell in general includes a magnetic tunnel junctionformed by a magnetic tunnel junction barrier layer featuring spinselective tunneling as well as a ferromagnetic free layer and areference layer on opposite sides of the magnetic tunnel junctionbarrier layer. To write information to the MRAM magnetic cell, thedirection of the magnetization of the ferromagnetic free layer may beadjusted by reorienting the direction of a magnetic moment of theferromagnetic free layer. For example, in spin transfer torque (STT)MRAM cells, a writing process can be performed by applying a currentthrough the tunnel junction of the MTJ structure, such that spinpolarized electrons tunneling through or being reflected from the tunneljunction may impart a torque on a magnetic moment of the ferromagneticfree layer and thereby adjust its magnetization direction with respectto the reference layer. Similarly, a read-out process of themagnetoresistive state of the STT-MRAM may be performed by applying acurrent through the tunnel junction of the MTJ structure and monitoringthe resulting voltage across the MTJ structure.

However, in the example of STT-MRAM the read-out process differs fromthe writing process mostly by the value of the injected current, suchthat a faulty read-out process may inadvertently affect the storedinformation. At the same time, the digital “1” and “0” states in MRAMcells generally differ by less than an order of magnitude, such that afast and precise measurement is usually associated with lower limits forthe detection current. Hence, robust and reliable operation of aplurality of MRAM cells in an MRAM memory commonly depends on precisecontrol over the magnetoresistive properties for each of the pluralityof MRAM cells during fabrication to minimize inadvertent informationread errors during access.

In particular, as the magnetoresistance properties of the MTJ structureare largely proportional to an area of the tunnel junction, accuratecontrol over the lateral dimensions of the MTJ structure is a desiredproperty for a corresponding fabrication method. Common approaches forprecisely defining miniaturized magnetoresistive elements includecomplex multi-patterning techniques, wherein lines of material withcontrollable width are arranged in a pattern above an MTJ material stackto define masking areas for subsequent etching steps. However, theseapproaches often include an extensive sequence of steps and cancontribute adversely to the production effort for fabricating MRAMmemory.

Examples described herein provide methods for fabricating semiconductordevices with controlled lateral dimensions to achieve robust andreliable operation of magnetoresistive memory devices. The methodprovides a precisely defined hard mask lateral shape for an MTJstructure associated with lithography-independent critical dimensionvariation. Further, examples of integrated circuits described hereininclude MTJ structures having corresponding shapes associated with lowtunnel junction area variance.

FIG. 1 schematically illustrates a portion of an MRAM cell referred toas the MRAM magnetic cell 102 formed on a substrate 100. The MRAMmagnetic cell 102 includes an MTJ structure 104 over a bottom electricalinterconnection layer 106, and further includes a top electricalinterconnection layer 108 over the MTJ structure 104. The top electricalinterconnection layer 108 and bottom electrical interconnection layer106 provide electrical connections to the MRAM magnetic cell 102, suchas a bit line access and a source line access that are used to read orwrite a state of the MRAM magnetic cell 102. The MTJ structure 104includes a bottom contact 110 over the bottom electrical interconnectionlayer 106, a magnetoresistive MTJ stack 112 over the bottom contact 110,a top contact 114 over the MTJ stack 112, and an MTJ cap 116 capping theMTJ structure 104.

The magnetoresistive MTJ stack 112 comprises a first magnetic interlayer118 and a second magnetic interlayer 122 separated by a magnetic tunneljunction (MTJ) barrier layer 120. In some examples, the first magneticinterlayer 118 can have a fixed or “pinned” magnetic orientation, whilethe second magnetic interlayer 122 has a variable or “free” magneticorientation, which can be switched between two or more distinct magneticpolarities that each represents a different data state, such as adifferent binary state. In other implementations, however, the MTJstructure 104 can be vertically “flipped”, such that the first magneticinterlayer 118 has a “free” magnetic orientation, while the secondmagnetic interlayer 122 has a “pinned” magnetic orientation. The firstmagnetic interlayer 118 or the second magnetic interlayer 122 comprisesa ferromagnetic layer comprising Fe, Co, Ni, FeCo, CoNi, CoFeB, FeB,FePt, FePd, or the like. The magnetic tunnel junction (MTJ) barrierlayer 120 may comprise, for example, magnesium oxide (MgO), aluminumoxide (e.g., Al2O3), NiO, GdO, Ta2O5, MoO2, TiO2, WO2, or the like. Tomaximize a magnetoresistive effect in the MTJ structure 104 or atransfer of an electron or hole's magnetic moment across the magnetictunnel junction barrier layer 120, the magnetic tunnel junction barrierlayer 120 may be in direct contact with the first magnetic interlayer118 and the second magnetic interlayer 122.

The first magnetic interlayer 118, the second magnetic interlayer 122,and the magnetic tunnel junction barrier layer 120 may respectively beformed using any suitable process, for example, by Physical VaporDeposition (PVD), DC PVD, RF PVD, Chemical Vapor Deposition (CVD),Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic LayerDeposition (ALD), pulse DC, High-Density Plasma CVD (HDPCVD), lowpressure CVD (LPCVD) or the like and may be formed in single or multiplelayers. For example, the magnetic interlayer 118, 122 having a pinnedmagnetic orientation may comprise a first pinned magnetic layer and asecond pinned magnetic layer, e.g. stacks of consecutive or alternatinglayers constituting first or second pinned magnetic layer stacks,separated by an antiferromagnetic coupling layer (not shown), such asruthenium (Ru) or Iridium (Ir). The magnetic interlayer 118, 122 havingthe pinned magnetic orientation may then comprise a further magnetic“reference” layer adjacent to the magnetic tunnel junction barrier layer120 and coupled to the first pinned magnetic layer or the second pinnedmagnetic layer.

The bottom contact 110 and the top contact 114 may comprise conductivelayers, such as conductive metals, e.g. layers comprising Cu, Co, Ta,Pt, Ti, TiN, W, Ru, Mo, Cr or the like. The bottom contact 110 and thetop contact 114 may provide electrical connections to the MTJ structure104 via the top electrical interconnection layer 108 and the bottomelectrical interconnection layer 106, respectively, such as to connectthe MTJ structure to active or passive devices formed in or on thesubstrate 100.

For example, as schematically illustrated in FIG. 2A, an MRAM cell 101may comprise a transistor 124 and an MRAM magnetic cell 102. The bottomcontact 110 of the MRAM magnetic cell 102 may be connected to a draincontact D of the transistor 124 through a bottom electrodeinterconnection layer 106, and a gate electrode G of the transistor 124may be connected to a word line 126 of an MRAM memory via conductiveinterconnection layers in the substrate 100 (layers below layer 106 notexplicitly shown). The top contact 114 of the MRAM cell 101 may beconnected to a bit line 128 of the MRAM memory via a top electrode 130of the MRAM cell 101. A source electrode S of transistor 124 may beconnected to a source line 132. However, in some examples the transistor124 may also be connected to the MTJ structure 104 via the top contact114 and the bottom contact 110 may be connected to the bit line 128. TheMRAM cell 101 may then be controlled in an STT-MRAM configuration bysending a current through the MTJ structure 104 via the bit line 128 andthe source line 132, controlled by an electrical potential applied tothe word line 126.

The schematically shown word line 126, bit line 128 or source line 132may be conductive interconnects of conductive interconnection layersembedded in or formed on the substrate 100 or embedded in an insulatingmaterial, for example. The conductive interconnection layers may includepatterned conductive layers and conductive vias extendingperpendicularly through the substrate and connecting the patternedconductive layers. The metallic interconnection layers may furthercomprise an Inter-Metal Dielectric (IMD) or an Inter-Layer Dielectric(ILD), which may include a dielectric material having a low dielectricconstant (k value) lower than 3.8, lower than about 3.0, or lower thanabout 2.5, for example. The insulating material may be formed ofPhosphosilicate glass (PSG), Borosilicate glass (BSG),Borophosphosilicate glass (BPSG), Fluorosilicate glass (FSG), Tetraethylorthosilicate (TEOS), hydrogenated silicon oxycarbide, acarbon-containing low-k dielectric material, Hydrogen silsesquioxane(HSQ), Methyl-silsesquioxane (MSQ), or the like.

While the example in FIG. 2A is illustrated in an STT-MRAMconfiguration, the MTJ structure 104 according to examples of thedisclosure may be applied to different MRAM configurations, such asmagnetic field-based MRAM, spin hall effect (SHE) or spin orbit torque(SOT) based MRAM, electric field assisted/voltage controlled magneticanisotropy (VCMA) MRAM, multi-level storage MRAM, or the like.

For example, FIG. 2B illustrates an example of an MRAM cell 101′ in anSOT-MRAM configuration. The SOT MRAM magnetic cell 102 comprises an MTJstructure 104 formed on an SOT layer 134 with a magnetic interlayer 118formed over the SOT layer 134 and the magnetic interlayer 118 may beconfigured as a magnetic free layer. A tunnel junction barrier layer 120is formed over the first magnetic interlayer 118 and a second magneticinterlayer 122 is formed over the tunnel junction barrier layer 120 andconfigured as a magnetic reference layer/magnetic pinned layer. A bitline 128 is connected to the second magnetic interlayer 122 via a topelectrode 130, a conductive cap 116, and a top contact 114.

The SOT layer 134 may be connected to a first terminal T2 of a firsttransistor 124 a and a first terminal T2 of a second transistor 124 barranged on opposite sides of the SOT layer 134 with respect to the MTJstack 112. A gate G of the first transistor 124 a may be connected to afirst word line 126 a and a gate G of the second transistor 124 b may beconnected to a second word line 126 b. A second terminal T2 of the firsttransistor 124 a may be connected to a first source line 132 a and asecond terminal T2 of the second transistor 124 b may be connected to asecond source line 132 b.

In the SOT-MRAM configuration, a writing process may be performed bysending a writing current through the SOT layer 134 from the firstsource line 132 a to the second source line 132 b. In the SOT layer 134spin-orbit coupling may then lead to deflection of electrons ofdifferent spin in different directions, such that a magnetic momentdirection change may be imparted on the first magnetic interlayer 118which depends on the direction of the current through the SOT layer 134.As an example, the SOT layer 134 may comprise tungsten (W), tantalum(Ta), platinum (Pt) or an alloy or compound composition, such as AuPt,formed as a thin layer having a thickness of about 3 nm to about 20 nm,e.g. having a thickness between 4 to 6 nm, to give an example. In someexamples, the SOT layer 134 is in direct contact with the first magneticinterlayer 118 or separated from the first magnetic interlayer 118 by athin interlayer (not shown) which does not prevent transfer of electronswith their spin magnetic moments from the SOT layer 134 to the firstmagnetic interlayer 118.

The state of the MTJ structure may then be read by passing a currentthrough the MTJ structure 104 from the bit line 128 to the first sourceline 132 a or to the second source line 132 b as discussed before withreference to the STT-MRAM configuration illustrated in FIG. 2A. Despitethe writing current path being largely decoupled from the MTJ structure104, a read-out process in the MRAM cell 101′ in the SOT-configurationmay still cause inadvertent magnetization switching due to the readcurrent or there may be interference of the wrong memory state due tolow read signal margin, such that accurate control over the lateraldimensions of the MTJ structure 104 remains a desired property for afabrication method of an MRAM cell 101′ in an SOT-MRAM configuration, orother MRAM configurations.

A plurality of MRAM magnetic cells 102 as shown in FIG. 1, 2A or 2B maybe arranged over the substrate 100 in rows and columns, e.g. on a squareor hexagonal lattice, which rows and columns may be associated withrespective word lines 126 and bit lines 128 to form an array of the MRAMmemory. To selectively read or write a state of the MRAM cell 101, 101′in the MRAM memory, appropriate electrical potentials may beconcurrently applied to the source line 132 and to a respective pair ofa word line 126 and a bit line 128 corresponding to a certain MRAMmagnetic cell 102. These electrical potentials may be selected such thata resulting current for reliably reading the state of the MRAM magneticcell 102 does not inadvertently switch a magnetization of theferromagnetic free layer. Hence, the magnetoresistive properties of theMTJ structure 104 of the MRAM cell 101, 101′ should be accuratelycontrolled during a fabrication process in order to avoid unreliablereading and simultaneously to limit inadvertent switching ofmagnetization due to falsely or inappropriately selected electricalpotentials.

For that purpose, the layers of the MTJ structure 104 may be formedconsecutively one over the other. Then a lateral dimension of the MTJstructure 104 and in particular the lateral area of the MTJ stack 112 ator close to the magnetic tunnel junction barrier layer 120 may bedefined by etching the sequence of layers with a hard mask having awell-defined lateral dimension. In particular, the mask may be formed todefine an intended critical dimension of the MTJ structure 104, such asa diameter or a width of the MTJ structure 104. The resulting structuremay then be a pillar-shaped MTJ structure 104 capped by the hard mask,which may serve as the MTJ cap 116, and having a contour correspondingto the contour of the hard mask associated with the critical dimension.

FIGS. 3 through 14 illustrate intermediate stages of the creation of anMTJ structure 104 in accordance with some examples. FIG. 3 illustratesproviding a substrate 100.

The substrate 100 may be formed of a semiconductor or insulatingsubstrate, such as a silicon substrate a silicon germanium substrate ora silicon on insulator (SOI) substrate, or the like. In some examples,the substrate 100 is a crystalline semiconductor substrate such as acrystalline silicon substrate, a crystalline silicon carbon substrate, acrystalline silicon germanium substrate, a III-V compound semiconductorsubstrate, or the like. In an example, the substrate 100 may comprisebulk silicon, doped or undoped, or an active layer of asilicon-on-insulator (SOI) substrate. Generally, an SOI substratecomprises a layer of a semiconductor material such as silicon,germanium, silicon germanium, or combinations thereof, such as silicongermanium on insulator (SGOI). Other substrates that may be used includemulti-layered substrates, gradient substrates, or hybrid orientationsubstrates.

In some examples, the substrate 100 may be processed to includeconductive features, such as active and passive devices, and conductiveinterconnection layers. The conductive interconnection layers of thesubstrate 100 may comprise the bottom electrical interconnection layer106 and may provide electrical connections to active devices, passivedevices or to the bottom contact 110 of the previously illustrated MRAMmagnetic cell 102 or a combination thereof.

In FIG. 4A an MTJ layer stack 200 is formed over the substrate 100. TheMTJ layer stack 200 comprises an insulating layer for a tunnel junctionbarrier layer 120 and a plurality of magnetic layers to form the firstmagnetic interlayer 118 and the second magnetic interlayer 122. Inaddition, the MTJ layer stack 200 may comprise additional layers tocontrol the magnetoresistive properties of the MTJ layer stack 200 aswell as the texture and anisotropy of the layers.

As shown in FIG. 4B, forming the MTJ layer stack 200 may compriseforming a bottom contact layer 210 over the substrate 100 having formedtherein a bottom electrical interconnection layer 106. Themagnetoresistive portion of the MTJ layer stack 200 is then formed byforming a first magnetic interlayer 218 over the bottom contact layer210, forming a tunnel junction barrier layer 220 over the first magneticinterlayer 218 and forming a second magnetic interlayer 222 over thetunnel junction barrier layer 220. The MTJ layer stack 200 may then becapped by a top contact layer 214 over the second magnetic interlayer222. It will however be appreciated that in practice, each of the layersillustrated in FIG. 4B may be subdivided into a plurality of differentlayers formed one over the other.

For example, in FIG. 4C an example of an MTJ layer stack 200 for formingan STT-MRAM cell 101 is illustrated. The MTJ layer stack 200 comprises abottom contact layer 210 which may be formed of conductive materials,such as metals, to electrically connect the MTJ structure 104 to thebottom electrical interconnection layer 106. The bottom contact layer210 may further comprise smoothing or seed layers for smoothing asurface of the underlying substrate 100 or bottom electricalinterconnection layer 106 or seeding a respective crystallographicorientation of overlying layers. For example, the bottom contact layer210 may comprise a layer of platinum in a crystallographic (111)orientation arranged over a layer of tungsten, or tantalum, and may beformed by any appropriate metal deposition process to a thicknessbetween about 5 nm and about 20 nm although other thicknesses may beused.

A first magnetic interlayer 218 may be formed over the bottom contactlayer 210 and may comprise a first portion 218 a, a second portion 218 band a third portion 218 c of magnetic material layers. The first portion218 a and the second portion 218 b may be separated by anantiferromagnetic coupling layer 219 and may form a syntheticantiferromagnet (SAF). As an example, the first portion 218 a and thesecond portion 218 b may comprise a sequence of stacked layerscomprising platinum and cobalt in a crystallographic (111) orientationand may be separated by an antiferromagnetic coupling layer 219comprising ruthenium or iridium to form a synthetic antiferromagnet witha perpendicular magnetization direction, i.e. wherein the direction ofmagnetization associated with the portions 218 a, 218 b of the syntheticantiferromagnet is oriented perpendicular with respect to the substrate100.

A third portion 218 c may then be arranged over the second portion 218 bto act as a pinned magnetic reference layer for the MTJ layer stack 200.The third portion may comprise a ferromagnetic material alloy such ascobalt iron (CoFe), nickel iron (NiFe), cobalt iron boron (CoFeB),cobalt iron boron tungsten (CoFeBW), or the like, and may contain thesame or different magnetic materials as the first portion 218 a or thesecond portion 218 b. Further, the third portion 218 c may have the sameor a different crystallographic orientation as the first portion 218 aor the second portion 218 b. For example, the third portion may comprisea layer of CoFeB having a crystallographic (100) orientation and may beseparated from the second portion 218 b by a texture breaking layer (notshown), such as a layer of tantalum, molybdenum, or tungsten. In someexamples, a net magnetization of the first portion 218 a, the secondportion 218 b and the third portion 218 c is zero or close to zero.

The magnetic tunnel junction barrier layer 220 may comprise magnesiumoxide (MgO) and may be formed over the third portion 218 c to athickness of between about 0.5 nm and about 3.5 nm thick, such as about1.5 nm thick, to form an insulating tunnel barrier between the firstmagnetic interlayer 218 in the second magnetic interlayer 222. Themagnetic tunnel junction barrier layer 220 should be thin enough thatelectrons are able to tunnel through the magnetic tunnel junctionbarrier layer 220 when a biasing voltage is applied across the MTJstructure 104.

The second magnetic interlayer 222 should be formed over the magnetictunnel junction barrier layer 220 and may comprise a first portion 222 aand a second portion 222 b which may be separated by a texture orinterface anisotropy inducing layer 223 a. The first portion 222 a andthe second portion 222 b may comprise the same or different layers andmay each comprise a ferromagnetic material alloy such as cobalt iron(CoFe), nickel iron (NiFe), cobalt iron boron (CoFeB), cobalt iron borontungsten (CoFeBW), or the like. As an example each of the first portion222 a and the second portion 222 b may be formed of layers of CoFeBhaving a crystallographic (100) orientation and may be formed to athickness of about 1-2 nm. The first portion 222 a and the secondportion 222 b may be separated by a layer of tantalum as an example ofthe texture or interface anisotropy inducing layer 223 a. The secondmagnetic interlayer 222 may be capped by an interface anisotropyinducing layer 223 b, such as a magnesium oxide interlayer. The topcontact 214 may then conductively cap the second magnetic interlayer 222with metallic layers, such as Ta, W, Ru, Mo, or the like, and may beelectrically connected to the second magnetic interlayer 222, whereinthe connection may be through the interface anisotropy inducing layer223 b.

However, it will be appreciated that the example of FIG. 4C isillustrative and is not intended to imply any limitation. For example,while in FIG. 4C, the first magnetic interlayer 218 comprises a pinnedmagnetic layer and the second magnetic interlayer 222 comprise a freemagnetic layer, the structure may equally be flipped, and other materialchoices for the contact materials, magnetic materials, insulatingbarriers, coupling materials and texture or anisotropy inducing/breakinglayers and their respective arrangement may be selected. Further insteadof a synthetic antiferromagnet formed by the first and second portions218 a, 218 b, an antiferromagnetic layer may be deposited adjacent tothe pinned magnetic reference layer, such as a metal alloy includingmanganese (Mn) and another metal(s) such as platinum (Pt), iridium (Jr),ruthenium (Ru), rhodium (Rh), nickel (Ni), palladium (Pd), iron (Fe),osmium (Os), or the like.

FIG. 5 illustrates forming a pattern definition stack 300 over thesubstrate 100 with the MTJ layer stack 200 (shown schematically as asingle layer). The pattern definition stack 300 comprises a transferlayer 310, an interlayer 320 arranged over the transfer layer 310, and apatterning layer 330 arranged over the interlayer 320.

The patterning layer 330 and the interlayer 320 should comprisedifferent materials, such that the interlayer 320 may be selectivelyetched with a higher etch rate than the patterning layer 330. In someexamples, the patterning layer 330 comprises a hard mask material forprocessing the interlayer 320. The patterning layer 330 may comprise anitride, such as silicon nitride, titanium nitride, tantalum nitride, orthe like. In some examples, the patterning layer 330 may be made of acomposition which includes tantalum, tungsten, chromium, ruthenium,molybdenum, silicon, germanium, or combinations thereof, as well asnitrides and/or oxides of these materials. The patterning layer 330 maybe formed using any suitable process, for example, by Physical vapordeposition (PVD), DC PVD, RF PVD, Chemical Vapor Deposition (CVD),Plasma Enhanced Chemical Vapor Deposition (PECVD), Atomic LayerDeposition (ALD), pulse DC, High-Density Plasma CVD (HDPCVD), lowpressure CVD (LPCVD) or the like, to a thickness between about 10 nm and50 nm, though other thicknesses may be used.

Accordingly, the materials of the interlayer 320 may be selected to beuniformly formed and to be etched selectively with a higher etch ratethan the material of the patterning layer 330 by a well-controlledisotropic etchant. For example, the patterning layer 330 may comprisepolysilicon (Si) and the interlayer 320 may comprise silicon oxide(SiO_(x)), in which case a buffered HF solution can be used to providethe desired selective etch. In some examples, the patterning layer 330may comprise silicon nitride (SiN) in which case a suitableconcentration of a buffered HF solution can be used to provide aselective etch. The interlayer 320 may be formed using any suitableprocess, for example, by PVD, RF PVD, CVD, PECVD, ALD, pulse DC, or thelike, to a thickness which may be larger than the critical dimension ofthe MTJ structure 104, such as between about 60 nm and 300 nm, thoughother thicknesses may be used depending on the intended scale of the MTJstructure 104.

The transfer layer 310 should be different from the interlayer 320 andmay be different from the patterning layer 330. In some examples, thetransfer layer 310 functions as an etch stop layer with respect to theinterlayer 320. Accordingly, the transfer layer 310 may include metal orsemiconductor material, such as an oxide, nitride, or carbide of a metalor semiconductor material different from the interlayer 320 andresistant to an etchant of the interlayer 320. Such materials mayinclude, for example, silicon nitride, aluminum nitride, aluminum oxide,silicon carbide, silicon carbide, and the like. The transfer layer 310may include multiple layers of the same or different materials. Thetransfer layer 310 may be formed by any suitable method, such as byPECVD or other methods such as HDPCVD, ALD, LPCVD, PVD, and the like toa thickness between about 20 nm and 60 nm, though other thicknesses maybe used.

In some examples, the transfer layer 310 comprises the same material asthe patterning layer 330. For example, the patterning layer 330 and thetransfer layer 310 may comprise silicon nitride or polysilicon, whereasthe interlayer 320 comprises silicon oxide.

FIG. 6 illustrates forming a first opening 340 in the patterning layer330 to expose an upper surface of the interlayer 320. The first opening340 may be formed by any suitable method. For example, the first opening340 may be formed by a photo-patterning process, using a patterned photoresist (not shown). The patterning layer 330 may then be etched totransfer a pattern of the patterned photoresist to the patterning layer330 and to expose the upper surface of the interlayer 320. The patternedphotoresist may be removed after exposing the upper surface of theinterlayer 320 or may remain on top of the patterning layer 330 toprocess the interlayer 320.

The first opening 340 may have a lateral dimension (width) larger thanthe intended critical dimension of the MTJ structure 104. In someexamples, the first opening 340 has a circular shape and a diameter ofthe first opening 340 is larger than the intended critical dimension,such as a diameter of the MTJ structure 104. For example, a lateraldimension of the first opening 340 may be between about 30 nm to about300 nm, or a ratio of the lateral dimension of the first opening 340 andthe intended critical dimension may be between about 1.5 and about 10.

FIG. 7 illustrates forming a cavity 350 in the interlayer 320 throughthe first opening 340. In some examples, the cavity 350 and the firstopening 340 are formed consecutively using the patterned photoresist(not shown). For example, an etchant may be used to consecutively etchthe patterning layer 330 according to a pattern in the patternedphotoresist and the interlayer 320 through the patterned photoresist andthe first opening 340. However, the patterned photoresist may also beremoved after forming the first opening 340, and the cavity 350 in theinterlayer 320 may be formed by etching the interlayer through the firstopening 340.

In some examples, etching the interlayer 320 may compriseanisotropically etching the interlayer 320 through the first opening 340to form a cylindrical cavity 350 extending into the interlayer 320.Hence, a shape of the first opening 340, such as the circular shapediscussed above, may be transferred into the interlayer 320. Thecylindrical cavity 350 may extend vertically into the interlayer 320,i.e. substantially perpendicularly with respect to the substrate 100.For example, the interlayer 320 may be etched with an anisotropicreactive ion etch (RIE) process or similar dry etch process to form ahigh aspect ratio hole or a plurality of holes in the interlayer 320.Depending on the aspect ratio produced by the anisotropic etch, thecylindrical cavity 350 extending vertically into the interlayer 320 mayalso be slightly cone-shaped but still be considered a cylindricalcavity 350 according to the present disclosure.

A depth of the cavity 350 into the interlayer 320 along the verticaldirection may be larger than the intended critical dimension of the MTJstructure 304 along a lateral direction. For example, the interlayer 320may be thicker than the intended critical dimension, e.g. a width of theMTJ structure 104, and the cavity 350 may expose an upper surface of thetransfer layer 310, as illustrated in FIG. 7.

FIG. 8 illustrates forming a recessed cavity 350 in the interlayer 320recessed with respect to the patterning layer 330. In the recessedcavity 350, a lateral surface 320 s of the interlayer 320 in the cavity350 is recessed with respect to a lateral surface 330 s of thepatterning layer 330 in the first opening 340 to a recess depth D. Inother words, an overhanging portion 330 o of the patterning layer 330may overhang the cavity 350 by the recess depth D, such that a width W1of the cavity 350 may differ from a width W2 of the first opening 340 bytwice the recess depth D.

The recessed cavity 350 may be obtained by etching the interlayer 320with an at least partially isotropic etchant through the first opening340, wherein an etch rate of the at least partially isotropic etchantfor the interlayer 320 is higher than an etch rate for the transferlayer 310 and the patterning layer 330. The interlayer 320 may be etchedwith a selective isotropic etch, such as a wet etch, vapor etch orisotropic plasma etch. For example, the interlayer 320 may comprise anoxide, e.g. silicon oxide, and the patterning layer 330 may comprisepolysilicon. A selective isotropic etch may comprise an oxide etch, e.g.hydrofluoric acid in vaporous, liquid, buffered, or diluted form,selected to negligibly etch the patterning layer 330 with respect to theinterlayer 320 during the etching of the interlayer 320 with the atleast partially isotropic etchant. Alternatively, the patterning layer330 may comprise a silicon nitride layer while the interlayer 320comprises silicon oxide with the isotropic wet etchant comprising ahydrofluoric (HF) acid-based solution with a pH adjusted to much morerapidly etch silicon oxide than it etches silicon nitride.

In some examples, the interlayer 320 is etched with the at leastpartially isotropic etchant through the first opening 340 to obtain arecess depth D corresponding to twice the intended critical dimensionfor the MTJ structure 104.

In FIG. 9, a conformal layer 360 is formed over the interlayer 320 andthe patterning layer 330 to fill the first opening 340. The conformallayer 360 may be formed of any material with a suitable homogenousconformal deposition process, which may conformally be deposited withinthe cavity 350 with a uniformly deposited thickness. For example, thehomogenous conformal deposition process may be an ALD, CVD or similarprocess that deposits a layer of material with uniform thickness onexposed surfaces of the interlayer 320 and the patterning layer 330. Insome examples, the conformal layer comprises silicon, such aspolysilicon, deposited conformally over the exposed surfaces of theinterlayer 320 and the patterning layer 330.

As illustrated in FIG. 9, the conformal layer 360 fills the firstopening 340 and leaves a pore 370 arranged between an upper surface ofthe patterning layer 330 and an upper surface of the transfer layer 310.As the first opening 340 is filled by the conformal layer 360, portionsof the conformal layer 360 deposited on opposite lateral surfaces of thepatterning layer 330 merge and the inner surfaces of the cavity 350 areno longer exposed, such that further deposition of the conformal layer360 in the space occupied by the pore 370 is prevented. The width W3 ofthe pore 370 may then correspond to a difference between the width W1 ofthe cavity 350 and the width W2 of the first opening 340. As theconformal layer 360 merges, the transport of material through theshrinking first opening 340 will be reduced such than the merger of thesurfaces may be complete near the top side of the merging films butincomplete closer to the bottom side of the cavity 350, leaving a smallgap near the bottom. In other words, a gap in the merger below apinched-off section may be formed with a shape like that of an invertedcone (not illustrated).

In some examples, the pore 370 comprises a cylindrical pore segment 370c, wherein the width W3 of the pore 370 is constant or substantiallyconstant over a vertical height of the cylindrical pore segment 370 c. Aconformal deposition generally smoothes edges, such that the pore 370may have a contour corresponding to a rounded contour of the recessedcavity 350. A cylindrical segment 370 c of the pore 370 may be obtainedby first anisotropically etching the interlayer through the firstopening 340 to form a cylindrical cavity 350 in the interlayer 320before isotropically etching the interlayer 320 and forming theconformal layer 360. Isotropically etching the interlayer 320 andforming the conformal layer 360 may preserve the shape of thecylindrical cavity 350, such that the contour of the cylindrical cavity350 may be transferred to the pore 370 in a central cylindrical segment370 c. To transfer the shape of the cylindrical cavity 350 to the pore370, when forming the conformal layer 360, the interlayer 320 may beformed with a thickness greater than twice the recess depth D, such thatthe pore 370 assumes a vertically elongated shape when the first opening340 is filled with the conformal layer 360. The central cylindricalsegment 370 c may then be primarily formed by portions of the conformallayer 360 conformally grown on the interlayer 320, as opposed toconformally grown on the patterning layer 330 or the transfer layer 310.

The cylindrical pore segment 370 c may have a circular shape induced bythe shape of the first opening 340 or may have a rounded shapecorresponding to a shape of the first opening 340 with edges rounded bythe conformal deposition of the conformal layer 360. As the depositionof the conformal layer 360 rounds the edges of the contour of therecessed cavity 350 when forming the pore 370, a contour of the pore 370may approach a circular shape independently of the contour of the cavity350 for increasing thickness of the conformal layer 360. As an example,when a conformal layer 360 is formed in a cavity 350 having a squarecontour, edges of the initially square contour may be rounded by mergingportions of the conformal layer 360, eventually approaching a circularcontour. Hence, to minimize a variation of the shape of the contour ofthe pore 370 during isotropic etching of the interlayer 320 or formingthe conformal layer 360, the first opening 340 may be formed with acircular shape.

In FIG. 10, the conformal layer 360 is etched with an anisotropic etchto form a transfer aperture 380 through the pore 370 exposing an uppersurface of the transfer layer 310. As the pore 370 is free of material,anisotropically etching the conformal layer 360 may transfer a shape ofthe pore 370 onto the transfer layer 310. In other words, a contour ofthe exposed portion of the upper surface of the transfer layer 310 maycorrespond to a contour of the pore 370. Hence, a width W4 of exposedportion of the upper surface of the transfer layer 310 may correspond tothe width W3 of the pore 370.

In FIG. 11 a second opening 390 is formed in the transfer layer 310using the pore 370. The second opening 390 may be formed byanisotropically etching the transfer layer 310 through the transferaperture 380. In some examples, the conformal layer 360 and the transferlayer 310 are etched consecutively with an anisotropic etch to transfera contour of the pore 370 onto the transfer layer 310 and to form thesecond opening 390 in the transfer layer 310 having a contourcorresponding to the contour of the pore 370.

As illustrated in FIG. 11 etching the transfer layer 310 may partiallyor fully remove the patterning layer 330. However, the conformal layer360, the transfer layer 310 and the patterning layer 330 may also beetched with a common etchant or the patterning layer 330 may persistthrough forming the second opening 390.

As further illustrated in FIG. 11, forming the second opening 390 mayexpose the magnetic tunnel junction (MTJ) layer stack 200 through thesecond opening 390. In some examples, the conformal layer 360 and thetransfer layer 310 are consecutively etched to transfer a contour of thepore 370 onto an upper surface of the MTJ layer stack 200.

In FIGS. 12A-C a hard mask layer 400 is deposited in the second opening390. In FIG. 12A, remaining material of the conformal layer 360 and theinterlayer 320 is removed prior to depositing the hard mask layer 400 inthe second opening 390. FIG. 12B shows an alternative example, whereinthe conformal layer 360 and the interlayer 320 are not removed prior todepositing the hard mask layer 400, and the hard mask layer 400 isinstead deposited in the second opening 390 and in the transfer aperture380. FIG. 12C shows a further example, wherein the patterning layer 330,the conformal layer 360 and the interlayer 320 are not removed prior todepositing the hard mask layer 400, and the hard mask layer 400 isinstead deposited in the second opening 390, in the transfer aperture380, and over the patterning layer 330.

The hard mask layer 400 may comprise any hard mask material such astitanium nitride, tantalum nitride, or the like. In some examples, thehard mask layer 400 may be made of a composition which includestantalum, tungsten, chromium, ruthenium, molybdenum, silicon, germanium,other MRAM compatible metals, or combinations thereof, or includingnitrides and/or oxides of these materials. For example, the hard masklayer 400 may comprise a conductive composition of MRAM compatiblemetals, e.g. a non-magnetic or refractory metal or metal compound, suchas tungsten or tantalum, to conductively cap an underlying MTJ layerstack 200. The material of the hard mask layer 400 should be differentfrom the material of the transfer layer 310. The hard mask layer 400 maybe formed using any suitable process, for example, by PVD, DC PVD, RFPVD, CVD, ALD, pulse DC, or the like, to fill the second opening 390.

In FIG. 13 a planarization process is performed to remove portions ofthe hard mask layer 400 to expose the transfer layer 310 and to form ahard mask 410 having a shape corresponding to the second opening 390 inthe transfer layer 310. In case the conformal layer 360 and theinterlayer 320 are not removed prior to depositing the hard mask layer400, e.g. as shown in FIGS. 12B, 12C, the planarization process may alsoremove remaining material of the conformal layer 360 or the interlayer320.

For example, a chemical mechanical polishing process may be performed toremove portions of the hard mask layer 400 overlying the transfer layer310, the interlayer 320, the patterning layer 330 or the conformal layer360. However, depending on the deposition process used for depositingthe hard mask layer 400, an isotropic or an anisotropic etch may also beused to expose the transfer layer 310 and to form a hard mask 410 havinga shape corresponding to the second opening 390 in the transfer layer310. For example, when the hard mask layer 400 is depositedsubstantially conformally over the transfer layer 310, an anisotropicetch may remove any portions of the hard mask layer 400 overlying thetransfer layer 310 before removing all of the hard mask material in thesecond opening 390.

In FIG. 13, the second opening 390 and the corresponding hard mask 410feature substantial perpendicular sidewalls with respect to substrate100, such that a contour defined through the pore 370 onto the uppersurface of the transfer layer 310 is transferred through the secondopening 390 onto the upper surface of the MTJ layer stack 200. However,in some examples, the hard mask 410 features an undercut originatingfrom a shape of the transfer aperture 380 or introduced by an etchprocess for forming the second opening 390 in the transfer layer 310.For example, the hard mask 410 may comprise a tapered lateral surfacedefining an undercut of the hard mask 410, wherein a taper of thelateral surface corresponds to a taper of the transfer aperture 380 oris derived therefrom. Alternatively or additionally, an etching processof the transfer layer 310 may introduce an undercut in the secondopening 390 which may also provide a tapered lateral surface to the hardmask 410.

In FIG. 14, the transfer layer 310 is removed and the MTJ layer stack200 is patterned using the hard mask 410. For example, an anisotropicetching process may be performed to remove the transfer layer 310 andportions of the MTJ layer stack 200 not covered by the hard mask 410.However, the transfer layer 310 may also be first removed using anetchant selective to the transfer layer 310, which may only negligiblyetch the hard mask 410, and consecutive anisotropic etching steps may beused to pattern the MTJ layer stack 200 using the hard mask 410. Theanisotropic etching steps transfer a contour of the hard mask to the MTJlayer stack 200 and allow forming a pillar-shaped MTJ structure 104having lateral dimensions defined by the shape of the hard mask 410.

As the width W4 of the hard mask 410 may be largely independent of adimension defined by the initial lithographic patterning for obtainingthe first opening 340, the lateral dimensions of the patterned MTJ layerstack 200 are substantially lithography-variation independent. Instead,the width W4 of the hard mask 410 may depend primarily on the recessdepth D introduced by the isotropic etching of the interlayer 320. Avariation of the area of the hard mask 410 may thus be controlled by avariation of the etch rate of the interlayer 320 and may be smaller than6% or smaller than 5%, such as smaller than 3%, as measured by dividingthe standard deviation of the area of the hard mask 410 by the meanvalue of the area of the hard mask 410 for a plurality of hard masks 410formed concurrently over the substrate 100.

In some examples, the width W4 of the hard mask 410 corresponds to thewidth W3 of the pore 370 or is derived therefrom. For example, the widthW4 of the hard mask 410 may be reduced or increased with respect to thewidth W3 of the pore 370 due to tapered sidewalls of the transferaperture 380 or the second opening 390, which may be induced by theanisotropic etch of the conformal layer 360 or the transfer layer 310.

FIGS. 15 through 17 illustrate intermediate stages of the formation ofan MTJ structure 104 using the hard mask 410 according to an example.FIGS. 18 through 21 illustrate intermediate stages of the creation ofthe formation of an MTJ structure 104 using the hard mask 410 accordingto another example including the intermediate formation of a spacer.

FIG. 15 illustrates a substrate 100 having formed thereon a bottomelectrical interconnection layer 106 and an MTJ layer stack 200 formedover the bottom interconnection layer 106. The hard mask 410 definedwith a lithography-variation independent formation process as describedwith reference to FIGS. 3-14 is formed over the MTJ layer stack 200.

The MTJ layer stack 200 comprises a bottom contact layer 210 to contactthe bottom electrical interconnection layer 106, a first magneticinterlayer 218 over the bottom contact layer 210, a magnetic tunneljunction barrier layer 220 over the first magnetic interlayer 218 and atop magnetic interlayer 222 over the tunnel junction barrier layer 220.A top contact layer 214 is arranged between the second magneticinterlayer 222 and the hard mask 410.

FIG. 16A illustrates patterning the MTJ layer stack 200 using the hardmask 410. The MTJ layer stack 200 may be patterned using an anisotropicetch to form a pillar-shaped MTJ structure 104 in a portion of the MTJlayer stack 200 covered by the hard mask 410. Thus, a contour of thehard mask 410 is transferred to the tunnel junction barrier layer (220in FIG. 15) and the magnetic interlayers (218, 222 in FIG. 15) to form apatterned tunnel junction having a contour corresponding to the contourof the hard mask 410, and a variation of the area of the magnetic tunneljunction may thus be smaller than 6% or smaller than 5%, such as smallerthan 3%, as measured by dividing the standard deviation of the area ofthe MTJ stack 112 by the mean value of the area of the MTJ stack 112 fora plurality of pillar-shaped MTJ structures 104 formed concurrently overthe substrate 100.

FIG. 16B shows a schematic perspective view of the pillar-shaped MTJstructure 104, wherein the hard mask 410 has a circular contour, and theMTJ structure 104 is accordingly formed pillar-shaped with a contour ofthe pillar-shaped MTJ structure 104 being circular, such as cone-shapedor circular cylinder-shaped. In some examples, the MTJ structure 104 isformed to feature perpendicular (i.e. out-of-plane) magnetization of thepinned and free magnetic layers 118, 122 on opposite sides of the tunneljunction barrier layer 120. A perpendicular magnetization may not relyon shape anisotropy of the MTJ structure 104 and may thus be combinedwith a circularly shaped MTJ structure 104.

In FIG. 17, the MTJ structure 104 is covered with or embedded in adielectric 420 and a top electrode 130 is formed to contact the hardmask 410. In some examples, the hard mask 410 comprises an MRAMcompatible metal, such as tungsten or tantalum, and the hard mask 410may serve as a conductive MTJ cap 116 of the MTJ structure 104. Hence,the MTJ structure 104 may be directly connected with a top electricalinterconnection layer 108, 128 via a top electrode 130 contacting thehard mask 410 as shown in FIG. 17. Contacting the MTJ structure 104 viathe hard mask 410 reduces interference with the MTJ structure 104 andtherefore provides a robust and reliable process to form pillar-shapedMTJ structures 104 with lithography-variation independent criticaldimension. However, in some examples, the hard mask 410 may be removedprior to electrically contacting the top contact 114 with the topelectrode 130, e.g. in a dual damascene metal interconnect fabricationprocess.

The dielectric 420 may be an Inter-Metal Dielectric (IMD) or anInter-Layer Dielectric (ILD), which may include a dielectric materialhaving a low dielectric constant (k value) lower than 3.8, lower thanabout 3.0, or lower than about 2.5, for example. The insulating materialmay be formed of PSG, BSG, BPSG, FSG, TEOS, hydrogenated siliconoxycarbide, a carbon-containing low-k dielectric material, HSQ, MSQ, orthe like. The top electrode 130 may be formed by any suitablelithography process to pattern the dielectric 420 and to expose the hardmask, and by performing a consecutive metal deposition process, such asby electro-plating, electroless plating, PVD, DC PVD, RF PVD, CVD, ALD,pulse DC, and the like.

FIG. 18 illustrates another example of an intermediate stage of aprocess to form an MTJ structure 104 based on the structure depicted inFIG. 15. In FIG. 18, the hard mask 410 is used as the mask for etchingan upper portion of the MTJ layer stack 200 over the tunnel junctionbarrier layer 220, such that an upper surface of the tunnel junctionbarrier layer 220 is exposed. In other words, FIG. 18 illustrates theresult of etching the upper portion of the MTJ layer stack 200 to exposethe tunnel junction barrier layer 220. However, in some other examples,the MTJ layer stack 200 is etched to expose an upper surface of a layeradjacent to the tunnel junction barrier layer 220, such as the magneticinterlayer 218.

In FIG. 19, insulating sidewall spacers 430 are formed over sidewalls ofthe upper portion of the MTJ layer stack 200. For example, an insulatingmaterial may be conformally deposited over the upper portion of the MTJlayer stack 200, the hard mask 410 and the tunnel junction barrier layer220, and the resulting structure may be etched with an anisotropic etchto form the insulating sidewall spacers 430. The insulating sidewallspacer 430 may cover the sidewalls of the upper portion of the MTJ layerstack 200 and may protect the upper portion of the MTJ layer stack 200or may prevent shorting through or shunting out the tunnel junctionbarrier layer 120 during subsequent processing steps. Note in this casethe active tunnel junction area of the magnetic tunnel junctionstructure determining the magnetoresistive properties of the magnetictunnel junction would correspond to the area of the upper portion of theMTJ layer stack 200, e.g., the area of the second magnetic interlayer122, and not to that of the extended barrier layer 220.

In FIG. 20, the hard mask 410 and the insulating sidewall spacer 430 areused to etch the lower portion of the MTJ layer stack 200 and to form apillar-shaped MTJ structure 104 in a portion of the MTJ layer stack 200covered by the hard mask 410 and the insulating sidewall spacer 430. Asthe active area of the tunnel junction barrier layer 120 largelycorresponds to the area covered by the second magnetic interlayer 122,an effective standard deviation of the area of the tunnel junction maythen again be smaller than 6% or smaller than 5%, such as smaller than3%, as measured by dividing the standard deviation of the active area ofthe magnetic tunnel junction by the mean value of the active area of themagnetic tunnel junction, which is the area of the top magneticinterlayer 122 at the tunnel junction barrier layer 120 in the case ofFIG. 20, for a plurality of pillar-shaped MTJ structures 104 formedconcurrently over the substrate 100. Those skilled in the art willappreciate that any real system may nonetheless feature a finiteeffective standard deviation of the area of tunnel junction, such as aneffective standard deviation of the tunnel junction greater than 0.1% orgreater than 1%.

Further, the circular hard mask 410 may feature a diameter correspondingto the recess depth D of the cavity 350 which can be independent of anyphotolithographic mask used in a fabrication process, and may thus alsohave a sub-lithographic dimension, such as being smaller than aphotolithographic resolution limit (photo-lithographic criticaldimension), e.g. smaller than 60 nm or smaller than 30 nm.

In some examples, the plurality of pillar-shaped MTJ structures 104 arespaced by a photo-lithographic critical dimension, and a ratio betweenthe diameter of the circular hard mask 410 and the distance betweenadjacent pillar-shaped MTJ structures 104 is between 1/1.5 to 1/10.

The photo-lithographic critical dimension may also be realized inconductive connection features to said pillar-shaped MTJ structures 104,such as a lateral via dimension of a conductive via forming anelectrical connection to an upper surface of the MTJ structures 104. Forexample, the lateral dimension of the top interconnection layer 108 orof a portion of the top electrical interconnection layer 108 may be ator above the photo-lithographic critical dimension, and the diameter ofthe circular hard mask 410 may be smaller than the photo-lithographiccritical dimension.

In some examples, a ratio between the diameter of the circular hard mask410 and the lateral dimension of the top electrical interconnectionlayer 108 is between 1/1.5 to 1/10.

In FIG. 21, the MTJ structure 104 is then covered with or embedded in adielectric 420 and a top electrode 130 is formed to contact the topportion of the MTJ structure 104 via the hard mask 410. However, in someexamples, the hard mask 410 may be removed prior to electricallycontacting the top contact 114 with the top electrode 130, or the hardmask 410 may be used as a top electrode.

FIG. 22 illustrates a methodology 1000 of forming an MTJ structure 104for an MRAM magnetic cell 102 in accordance with some examples. Althoughthis method and other methods illustrated and/or described herein areillustrated as a series of steps, acts or events, it will be appreciatedthat the present disclosure is not limited to the illustrated orderingor acts. Thus, in some examples, the acts may be carried out indifferent orders than illustrated, and/or may be carried outconcurrently. Further, in some examples, the illustrated acts or eventsmay be subdivided into multiple acts or events, which may be carried outat separate times or concurrently with other acts or sub-acts. In someexamples, some illustrated acts or events may be omitted, and otherun-illustrated acts or events may be included.

Acts 1002 and 1004 can result in, for example, the structure previouslyillustrated in FIG. 5 in some examples and includes forming a patterndefinition stack 300 over an MTJ layer stack 200 on a substrate 100. At1006, a first opening 340 is formed in the patterning layer 310 of thepattern definition stack 300 as illustrated in FIG. 6 or 7. At 1008, arecessed cavity 350 is formed in the interlayer 320 of the patterndefinition stack 300 by anisotropically etching the interlayer 320 withan at least partially isotropic etchant through the first opening 340,wherein the at least partially isotropic etchant selectively etches theinterlayer 320 which may result in the example of FIG. 8. At 1010, aconformal layer 360 is formed over the interlayer 320 and the patterninglayer 310 to fill the first opening 340 which may result in the exampleschematically depicted in FIG. 9. At 1012, the conformal layer 360 isetched anisotropically to form a second opening 390 in the transferlayer 310 which may result in the intermediate stages illustrated inFIGS. 10, 11. At 1014, a hard mask material 400 is deposited in thesecond opening 390 in order to define a critical dimension for the MTJlayer stack 200 which may result in any of the examples of FIGS. 12A-C,to form a hard mask 410 as illustrated in the example of FIG. 13. At1016, the MTJ stack 200 is patterned using the hard mask material 400deposited in the second opening 390 to form an MTJ structure 104 whichmay result in the examples illustrated in FIGS. 14-21.

Some examples relate to a method for forming a semiconductor device. Themethod comprises forming a pattern definition stack over a substrate,the pattern definition stack comprising a transfer layer, an interlayerarranged over the transfer layer, and a patterning layer arranged overthe interlayer. The method further comprises forming a first opening inthe patterning layer to expose an upper surface of the interlayer. Themethod further comprises etching the interlayer with an at leastpartially isotropic etchant through the first opening, wherein an etchrate of the at least partially isotropic etchant for the interlayer ishigher than an etch rate for the transfer layer and the patterninglayer. The method further comprises forming a conformal layer over theinterlayer and the patterning layer to fill the first opening, andetching the conformal layer and the transfer layer with an anisotropicetch to form a second opening in the transfer layer. The method alsocomprises depositing a hard mask material in the second opening.

Some examples relate to a method for forming a magnetoresistiverandom-access memory (MRAM) cell. The method comprises forming amagnetic tunnel junction (MTJ) stack over a substrate, the magnetictunnel junction (MTJ) stack comprising a first magnetic interlayer, atunnel junction barrier layer over the first magnetic interlayer and asecond magnetic interlayer over the tunnel junction barrier layer. Themethod further comprises forming a transfer layer over the magnetictunnel junction stack, forming an interlayer over the transfer layer,forming a patterning layer over an upper surface of the interlayer, andforming a first opening in the patterning layer to expose the uppersurface of the interlayer. The method further comprises etching theinterlayer with an at least partially isotropic etchant through thefirst opening, wherein an etch rate of the at least partially isotropicetchant for the interlayer is higher than an etch rate for the transferlayer and the patterning layer to form a recessed cavity in theinterlayer. The method further comprises and forming a conformal layerover the interlayer and the patterning layer to fill the first openingand to form a pore in the cavity, and etching the conformal layer andthe transfer layer with an anisotropic etch to transfer a lateraldimension of the pore onto the transfer layer and to form a secondopening in the transfer layer. The method further comprises depositing ahard mask material in the second opening.

Some examples relate to an integrated circuit. The integrated circuitcomprises a semiconductor substrate, a bottom electrode over thesubstrate, a circular magnetic tunneling junction (MTJ) disposed over anupper surface of bottom electrode, and a circular top electrode disposedover an upper surface of the magnetic tunneling junction. The circulartop electrode is concentric to the circular magnetic tunneling junction,and a diameter of the circular magnetic tunneling junction is smallerthan 60 nm or smaller than 30 nm.

It will be appreciated that in this written description, as well as inthe claims below, the terms “first”, “second”, “second”, “third” etc.are merely generic identifiers used for ease of description todistinguish between different elements of a figure or a series offigures. In and of themselves, these terms do not imply any temporalordering or structural proximity for these elements, and are notintended to be descriptive of corresponding elements in differentillustrated examples and/or un-illustrated examples. For example, “afirst magnetic layer” described in connection with a first figure maynot necessarily correspond to a “first magnetic layer” described inconnection with another figure, and may not necessarily correspond to a“first magnetic layer” in an un-illustrated example.

The foregoing outlines features of several examples so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the examples introduced herein. Thoseskilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for forming a semiconductor device, themethod comprising: forming a pattern definition stack over a substrate,the pattern definition stack comprising a transfer layer, an interlayerarranged over the transfer layer, and a patterning layer arranged overthe interlayer; forming a first opening in the patterning layer toexpose an upper surface of the interlayer; etching the interlayer withan at least partially isotropic etchant through the first opening,wherein an etch rate of the at least partially isotropic etchant for theinterlayer is higher than an etch rate for the transfer layer and thepatterning layer; forming a conformal layer over the interlayer and thepatterning layer to fill the first opening; etching the conformal layerand the transfer layer with an anisotropic etch to form a second openingin the transfer layer; and depositing a hard mask material in the secondopening.
 2. The method according to claim 1, the method furthercomprising: anisotropically etching the interlayer through the firstopening to form a cylindrical cavity extending into the interlayerbefore etching the interlayer with the at least partially isotropicetchant.
 3. The method according to claim 2, wherein forming thecylindrical cavity exposes an upper surface of the transfer layer. 4.The method according to claim 2, wherein etching the interlayer with theat least partially isotropic etchant recedes a lateral surface of theinterlayer in the cylindrical cavity with respect to a lateral surfaceof the patterning layer in the first opening over a recess depth, andwherein etching the conformal layer and the transfer layer with ananisotropic etch forms the second opening in the transfer layer having awidth corresponding to twice the recess depth.
 5. The method accordingto claim 1, wherein forming the conformal layer over the interlayer andthe patterning layer forms a pore arranged between an upper surface ofthe patterning layer and an upper surface of the transfer layer, thepore comprising a cylindrical pore segment; and wherein the methodfurther comprises: etching the transfer layer with the anisotropic etchto form the second opening in the transfer layer, a contour of thesecond opening corresponding to a contour of the cylindrical poresegment.
 6. The method according to claim 5, wherein etching theconformal layer and the transfer layer with an anisotropic etch to formthe second opening comprises: etching the conformal layer to expose anupper surface of the transfer layer corresponding to the contour of thecylindrical pore segment.
 7. The method according to claim 1, wherein athickness of the interlayer in the pattern definition stack is largerthan a width of the second opening.
 8. The method according to claim 1,the method further comprising: forming a magnetic tunnel junction (MTJ)stack over the substrate, the magnetic tunnel junction stack comprisinga magnetic tunnel junction barrier layer; and wherein the patterndefinition stack is formed over the magnetic tunnel junction stack. 9.The method according to claim 8, the method further comprising: exposingthe magnetic tunnel junction (MTJ) stack through the second openingbefore depositing the hard mask material.
 10. The method according toclaim 1, wherein the hard mask material is a metallic material.
 11. Themethod according to claim 1, wherein the hard mask material comprisestungsten (W) or tantalum (Ta).
 12. The method according to claim 1, themethod further comprising: performing a planarization process afterdepositing the hard mask material in the second opening to form a hardmask in the second opening comprising the hard mask material.
 13. Amethod for forming a magnetoresistive random-access memory (MRAM) cell,the method comprising forming a magnetic tunnel junction (MTJ) stackover a substrate, the magnetic tunnel junction (MTJ) stack comprising afirst magnetic interlayer, a tunnel junction barrier layer over thefirst magnetic interlayer and a second magnetic interlayer over thetunnel junction barrier layer; forming a transfer layer over themagnetic tunnel junction stack; forming an interlayer over the transferlayer; forming a patterning layer over an upper surface of theinterlayer; forming a first opening in the patterning layer to exposethe upper surface of the interlayer; etching the interlayer with an atleast partially isotropic etchant through the first opening, wherein anetch rate of the at least partially isotropic etchant for the interlayeris higher than an etch rate for the transfer layer and the patterninglayer to form a recessed cavity in the interlayer; forming a conformallayer over the interlayer and the patterning layer to fill the firstopening and to form a pore in the recessed cavity; etching the conformallayer and the transfer layer with an anisotropic etch to transfer alateral dimension of the pore onto the transfer layer and to form asecond opening in the transfer layer; depositing a hard mask material inthe second opening.
 14. The method according to claim 13, whereinforming the magnetic tunnel junction stack comprises: forming a bottomcontact layer over the substrate; forming the first magnetic interlayerover the bottom contact layer; forming the tunnel junction barrier layerover the first magnetic interlayer; forming the second magneticinterlayer over the tunnel junction barrier layer; forming a top contactlayer over the second magnetic interlayer.
 15. The method according toclaim 14, further comprising forming a spin orbit torque material layerover the substrate; and forming the first magnetic interlayer over thespin orbit torque material layer.
 16. The method according to claim 13,wherein the method further comprises: etching the transfer layer to forma hard mask comprising the hard mask material; and anisotropicallyetching the magnetic tunnel junction stack to form a magnetic tunneljunction structure.
 17. The method according to claim 13, whereinanisotropically etching the magnetic tunnel junction stack to form anMTJ structure comprises anisotropically etching the second magneticinterlayer to expose the tunnel junction barrier layer; forming aconformal spacer layer over sidewalls of the second magnetic interlayer.18. A method, comprising: receiving a semiconductor substrate; forming abottom electrode over the semiconductor substrate; forming a magnetictunneling junction (MTJ) disposed over an upper surface of the bottomelectrode; and forming a circular top electrode disposed over an uppersurface of the MTJ; wherein a sidewall of the circular top electrodecomprises a tapered lateral surface defining an undercut of the circulartop electrode, such that a top surface of the circular top electrode hasa first diameter and a lower portion of the circular top electrodecorresponding to the undercut and corresponding to a top surface of theMTJ has a second diameter that is smaller than 60 nm or smaller than 30nm.
 19. The method of claim 18, wherein the MTJ comprises a firstmagnetic interlayer and a second magnetic interlayer separated by a MTJbarrier layer, and the MTJ barrier layer comprises magnesium oxide,aluminum oxide, NiO, GdO, Ta2O5, MoO2, TiO2, or WO2.
 20. The method ofclaim 18, wherein forming the MTJ comprises: forming a transfer layerover an MTJ stack; forming an interlayer over the transfer layer;forming a patterning layer over an upper surface of the interlayer;forming a first opening in the patterning layer to expose the uppersurface of the interlayer; etching the interlayer with an at leastpartially isotropic etchant through the first opening, wherein an etchrate of the at least partially isotropic etchant for the interlayer ishigher than an etch rate for the transfer layer and the patterning layerto form a recessed cavity in the interlayer; forming a conformal layerover the interlayer and the patterning layer to fill the first openingand to form a pore in the recessed cavity; etching the conformal layerand the transfer layer with an anisotropic etch to transfer a lateraldimension of the pore onto the transfer layer and to form a secondopening in the transfer layer; depositing a hard mask material in thesecond opening, wherein an outer perimeter of the hard mask material hasthe second diameter.